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larvae swimming about on the coast of Holland, and some by the 

 middle of the month had bored into the wood and .attained the 

 adult Teredo form, though still very small, while others in Sep- 



three weeks for them to complete their metamorphosis. Verrill 

 states that the Teredo navalis on the coast of New England "pro- 

 duces its young in .May, and probably through the greater part or 

 all of the summer." Quatrefages says that the Teredos die during 

 the winter succeeding their birth. 



Keferstein tells us that some lamellibranchs attain their growth 



thought to live from ten to twelve years, while Tridacna gigantea 

 probably lives from sixty years to a century. 



The time of spawning usually takes place in summer. The 

 edible mussel (Mytilus edulis) and different species of Venus 

 are found with eggs and embryos among the gills from March till 

 May, on the coast of Holland and France, while Pholas and Fan- 

 dura and most other genera breed from July until September. On 

 the Sicilian coast, according to Poli, My a and Solen breed early 

 in spring ; Pholas, Chama, Venus, Donax, Anomia, Tellina and 

 Maetra in summer; Mytilus edulis from October to December. 



We have seen that the Lamellibranchs pass through a true 



directly comparable with the veliger state of most Cephalophora. 

 In after life the "head" of the bivalve, i.e. the oral and preoral 



the veli-er. diminishes greatly in "size and importance, becoming 

 finally merged with the postoral region and represented simply 

 by the palpi and foot, the mouth-opening being situated at or 

 near the extremity of the body, so that the old terra Acephala 



the large and well developed head of the snails (Cephalophora) 

 and cuttle-fishes (Cephalopoda). 

 The summary of changes is usually as follows : 



1. Egg fertilized by tailed spermatic particles. 



2. Morula. 



3. Gastrula. (Observed in a very few cases.) 



4. Veliger (Cephalula). In Unio and Cyclas wholly or mostly 

 suppressed. 



5. Adult Lamellibranch. 



